Combine the flour, yeast, salt, sugar, butter, dry milk, potato flour, water, and egg in the bowl of a standing mixer fitted with the dough hook and knead until smooth and elastic, about 5-7 minutes. You can also make this by hand or in a bread machine. Adjust the dough's consistency with additional flour or water as needed but remember, the more flour you add, the heavier and drier your buns will be. Transfer the dough to a lightly floured work surface and shape into a ball. Lightly spray the inside of a large bowl with non-stick cooking spray. Place the dough in the bowl and loosely cover with a kitchen towel or plastic wrap and place in a draft-free space to rise until doubled in size, about 1 to 1 1/2 hours.

Transfer the dough to a lightly floured work surface. Gently deflate the dough and divide into 8 equal portions. Roll each piece of dough into a smooth ball and flatten into a 3 1/2-inch disc. Transfer discs to a parchment lined baking sheet spacing them 2 1/2-inches apart. Cover lightly with a kitchen towel and allow to rise for 1 to 1 1/2 hours in a warm, draft-free space.

Preheat the oven to 400º F. In a small bowl, whisk together the remaining egg and one tablespoon of water. Brush the top of the buns with the egg wash and sprinkle with sesame seeds or poppy seeds if using. Bake the buns until golden brown, about 15 minutes. Transfer to a wire rack to cool completely. Slice and use for your favorite burger recipe. Enjoy!